Lubricating oil is used in all instruments and machines including friction sites. This is because lubricating oil has important roles in suppressing friction, abrasion, seizure, and the like as much as possible to prolong the life of the instruments and machines. However, the various performances demanded of lubricating oil depend on the types of instruments and machines or the intended purposes, and in particular, reduction of friction under severe conditions in which high contact pressures are applied is considered to be very difficult in the lubrication field.
Lubricating oil performance is evaluated by various methods including test methods using an actual machine and test methods using a model machine for testing friction sites assumed from an actual machine. Although it is preferable to use an actual machine to finally measure accurate practical performance, tests using actual machines require an immense amount of time and cost and hence are impracticable for initial performance evaluation. Accordingly, as a first evaluation, evaluations using a model machine for testing a friction site assumed from an actual machine are employed in many cases. Model tests for investigating the degrees of friction, abrasion, seizure, and the like are classified into three types depending on contact types, i.e., point contact, line contact, and surface contact. Typical examples of tests based on the point contact include high-speed four-ball friction tests, ball-on-disc friction and wear tests, and ball-on-plate reciprocating wear tests, typical examples of tests based on the line contact include block-on-ring friction tests and two-roller tests, and typical examples of tests based on the surface contact include ring-on-disc friction and wear tests and block-on-plate friction and wear tests. Of those, the tests based on the point contact are considered as the most severe test because a contact pressure higher than the line contact and the surface contact is applied.
In general, out of existing friction-reducing agents, organic molybdenum compounds are well known to have high friction-reducing effects (Patent Literature 1 to 4). Organic molybdenum compounds are considered to form a molybdenum disulfide film on a sliding surface on which metals come in contact with each other, such as a boundary lubrication area, that is, apart to which some degree of temperature and load are applied, to exhibit a friction-reducing effect, and the effect has been found in all lubricating oils, such as engine oils. However, organic molybdenum compounds do not exhibit friction-reducing effects under all conditions. Under severe conditions in which a high contact pressure is applied, such as point contact, it may be difficult to reduce the friction because the effect is lowered. Accordingly, in the market, there is a strong desire for the development of an additive for a lubricating oil that exhibits a friction-reducing effect effectively under all conditions.